Oreos are stocked in every supermarket and corner store and are instantly recognisable to children across the country.

They are also a tempting target for a lawsuit. A British-born lawyer called Stephen Joseph has filed suit against Kraft Foods Inc, the manufacturers, alleging not only that the biscuits are unhealthy, but that the company engages in "fraudulent and deceptive marketing".

His actions are well-timed. Anxiety about the side-effects of junk food, such as obesity and childhood diabetes and heart disease, are growing in America.

At first glance his suit, which would have Oreos removed from sale throughout the state of California, seems no different from recent litigation aimed at McDonald's and other fast-food giants. In those cases, it was argued that food manufacturers and retailers should take financial responsibility for the health disasters caused by their products.

But Mr Joseph is not primarily concerned with obesity. The word does not even appear in the 13-page suit he filed earlier this month in the Marin County Superior Court, north of San Francisco. Rather, his attention is focused on one ingredient: hydrogenated oil or "trans fat". The ill effects of trans fats are only just beginning to be understood, even though they are present in an estimated 40 per cent of products on US supermarket shelves.

They are also extremely common in Britain, where public awareness is even lower.

"This is very different from suing McDonald's," Mr Joseph said. "Everyone knows junk food is bad for you. But very few people know about trans fat and what it does to your body.

"Trans fat is not labelled. Food manufacturers have been getting away with this for many many years. It says 'partially hydrogenated' on the label, but nobody knows what that means."

What it means is that oil has been treated with hydrogen at high temperatures – a process that alters its molecular structure in ways that improves its shelf life and that of the products in which it is used.

According to Mr Joseph and a growing body of scientific opinion, it also exposes consumers to an increase in "bad" cholesterol intake that can lead to type-2 diabetes and heart disease. As he puts it: "Trans fats are placed into food to increase shelf life, but they also decrease human life."

Trans fat crops up in all kinds of products, not all of them obviously unhealthy. Sliced breads, margarine and other spreads, frozen waffles and pancakes, even low-fat muesli contain trans fat. Mr Joseph's suit is based on Californian public interest law that specifically protects consumers in cases where common products are thought to be safe but turn out not to be.

Oreos are the perfect target, not just because they are well known – around 11 billion are consumed each year – but because the company markets them heavily to children and even claims they are healthy.

Kraft recently opposed a proposal to include trans fat in food labelling, saying this would "mislead consumers and may result in food choices inconsistent with public health goals." Mr Joseph described that argument as "an insult to everyone's intelligence".

It's not clear how many people truly believe Oreos are good for you – Robert Redford once said that health food was good for the conscience "but Oreos taste a hell of a lot better". But their status as a cultural icon is uncontested. Some schoolchildren learn geometry by measuring the circumference of an Oreo cookie. "Oreo" is also one of the most common answers in American crosswords.

Last summer, a study by the National Academy of Sciences concluded "that no amount of trans fat was safe". As a result, the Food and Drug Administration now recommends that "intake of trans fats should be as low as possible". Because of heavy lobbying by the food industry, however, there is no obligation to mention it in food labelling, or to explain what its effects might be.

That is where Mr Joseph's lawsuit comes in. He is not seeking financial gain for himself or any particular group of plaintiffs. Rather, he is hoping to force Kraft and every other manufacturer to switch to safer forms of fat such as palm oil. The academy's report has already prompted Frito Lay, America's leading manufacturer of potato crisps, to switch to trans fat-free corn oil.

McDonald's has also announced it is cutting – but not eliminating – the trans fat content of its French fries.

What Mr Joseph is seeking is the legal mechanism that will put the food industry on the ropes in the same way that litigation against the tobacco industry has pressured leading firms such as Philip Morris.

Mr Joseph's interest in trans fat began not in the United States but in Britain, where his stepfather Arnold Isaacs developed type-2 diabetes and eventually died two years ago of coronary disease despite a lifetime of what he believed to be healthy eating.

Mr Joseph and his mother, Joyce Isaacs, believe it was margarine and a particular brand of sliced bread – both made with hydrogenated oil – that caused him to fall ill and die. They acknowledge they won't be able to prove it, but he is the emotional spur that pushed them into broader public advocacy.

Mrs Isaacs, who lives in Altrincham, Cheshire, said:"People in England are very trusting. They think if a product is in a big supermarket then it must be all right. But it's not."

She said she had lobbied her local branch of Marks & Spencer, only to be greeted with a "blank face" by the manager. Almost nobody she knows had heard of trans fat until she told them about it.

Research into the exact health effects of trans fat is still preliminary and uncertain. In the United States, the Food and Drug Administration has estimated that better labelling on food products could prevent between 7,600 and 17,100 cases of coronary heart disease each year and between 2,500 and 5,600 deaths.

Kraft has made no comment on his lawsuit. They have until early June to file a formal response, at which point the judge will decide whether the case has sufficient merit to move forward to trial.

FAST-FOOD LEGAL BATTLES

Months ago, a federal judge in New York threw out a lawsuit that blamed McDonald's hamburger chain for obesity, diabetes and other health problems in adults and children.

A lawyer claimed that a 123kg (20 stone) man did not know that food at McDonald's, Wendy's, Kentucky Fried Chicken and Burger King could be unhealthy.

But the judge said the plaintiffs did not demonstrate McDonald's products "involve a danger that is not within the common knowledge of consumers". The lawyer has filed an amended lawsuit accusing McDonald's of deceptive advertising. In 2001, students from George Washington University filed a lawsuit against McDonald's Corp accusing them of failing to disclose that French fries were prepared with beef fat. McDonald's settled by agreeing to pay $10m (£6.2m) to groups representing vegetarians, Hindus and Sikhs and organisations promoting Jewish dietary laws and children's welfare issues.

Stella Liebeck, 79, won $480,000 (£300,000) for third-degree burns after she spilt a cup of coffee in her lap in New Mexico in 1994. Evidence at the trial showed McDonald's knew of 700 other burns cases.